Photovoltaic devices utilize specific conductivity characteristics of materials generally referred to as semiconductors, whereby solar energy or radiation is converted to useful electrical energy. This conversion results from the absorption of photon energy in the active region of the cell, whereby some of the absorbed energy causes the generation of electron-hole pairs. The energy required for the generation of electron-hole pairs in a semiconductor material is referred to as the band gap energy and generally is the minimum energy required to excite an electron from the valence band to the conduction band.
There are two principal measures of the utility of photovoltaic devices. First is the efficiency of the device, which is an ascertainable percentage of the total photon energy converted to useful electrical energy. High efficiency photovoltaic devices made of crystalline materials maximize efficiency by minimizing internal lattice defects. The second measure of the utility of a photovoltaic device is its cost. Single crystal devices are complex and costly to produce, and do not readily lend themselves to high volume production.
One approach to reducing the cost of photovoltaic devices is to utilize polycrystalline thin film materials and a heterojunction. A heterojunction refers to the active junction formed at the interface between two dissimilar materials, such as cadmium sulfide and cadmium telluride as taught by Basol, et al. in U.S. Pat. No. 4, 388,483. Basol, et al. described thin-film heterojunction photovoltaic cells wherein the active layer comprises at least one of the metal elements of Class IIB of the Periodic Table of Elements and one of the non-metal elements of Class VIA of the Periodic Table of Elements. One feature of such photovoltaic devices is the use of extremely thin film active layers. As an example, Basol, et al. utilized a cadmium sulfide layer on the order of 0.02-0.05 micrometer thick and a cadmium telluride layer on the order of about 1.3 micrometers thick. While such economy of material has obvious advantages, it has also presented an unexpected concern with respect to current collection.
Such thin film photovoltaic devices typically comprise an optically transparent substrate through which radiant energy enters the device, a first semiconductor layer formed on the transparent substrate, a second semiconductor layer of opposite conductivity type than the first semiconductor layer forming a junction with the first layer, and a conductive film back contact. When the substrate is not electrically conductive, then a transparent electrically conductive film is disposed between the substrate and the first semiconductor layer so as to function as a front contact current collector; this front contact generally is a layer of a transparent conductive oxide. Transparent conductive oxides, such as indium tin oxide, indium oxide, zinc oxide, and tin oxide are not efficient current collectors in cells of any appreciable size, that is, greater than about one square centimeter, due to their inherent resistivity, which is on the order of about 10 ohms per square. The transparent conductive layer must be supplemented with more efficient current collection means such as a front contact current collector grid. This grid comprises a network of relatively low resistivity material that collects electrical current from the transparent conductive layer and efficiently channels the current to a central current collector.
Front contact current collector grids may be formed by various known processes such as vapor deposition and electrodeposition. Electrodeposition processes are especially preferred since, when coupled with masking techniques, are simple, cost effective techniques adaptable to scale-up.
Front contact current collector grids are generally made of materials such as copper, gold, and silver. Since the grid material is not optically transparent, the presence of the grid will lower the overall efficiency of the photovoltaic device. To minimize this disadvantage, current collector grids are designed to cover as little active surface area as possible. One way in which this is done is by forming extremely narrow gridlines in relation to the active surface area of the photovoltaic device.
Generally, a front contact current collector grid is deposited onto a transparent conductive layer and followed by subsequent depositions of the active semiconductor layers. This procedure has several drawbacks when applied to the formation of a front contact current collector grid by electrodeposition. One such drawback is the tendency of the current collector grid material to separate from the transparent conductive layer. If the gridline has a relatively large thickness, inherent stresses may favor separation of the gridlines from the transparent conductive layer to relieve such stresses. This separation may be aggravated by the subsequent processing steps used to form the photovoltaic device, especially by subsequent electrodeposition and heat treatment process steps. Such separation of the current collector grid from the transparent conductive layer may also occur with time, which is an undesirable occurrence in photovoltaic devices intended to function for on the order of about twenty years.
What is needed in the area of efficient current collection in thin film photovoltaic devices is a stable bond between the electroplated front contact current collector grid and the transparent conductive layer that does not give rise to the above-described drawback.
It is therefore one object of the present invention to provide an electroplated front contact current collector grid for photovoltaic devices, which current collector grid is not accompanied by the above-identified shortcomings.
It is another object of the present invention to provide a photovoltaic device having a stable electroplated front contact current collector grid incorporated therein.
It is yet another object of the present invention to provide a method for the formation of a photovoltaic device that has a stable electroplated front contact current collector grid incorporated therein.
These and additional objects of the present invention will become apparent to one skilled in the art from the below description of the invention and the appended claims.